The Government is on collision course with postal workers after confirming it is pressing ahead with controversial plans to privatise the Royal Mail, sparking claims that it is "selling off the family silver".

Ministers said it was a "very exciting" day as details of the share sale were announced but they were attacked by unions and opposition politicians.

The scale of anger was highlighted by a noisy reception given to Royal Mail chief executive Moya Greene as she addressed a meeting in Birmingham of more than 1,500 local officials of the Communication Workers Union (CWU). She was booed, heckled and greeted with laughter when she defended the sell-off.

The union will now press ahead with balloting its members for strike action in protest at issues linked to privatisation. Industrial action could start by mid-October, just as potential investors are being courted to buy shares.

Members of the public will be able to apply for shares - at a minimum of £750 - as well as institutional investors. The Government confirmed that 10% of shares will be given to 150,000 Royal Mail workers. Analysts expect the sell-off will make up to £3 billion.

CWU general secretary Billy Hayes said: "This isn't about what's best for the Royal Mail, it's about vested interests of government ministers' mates in the City. Privatisation is the worst way to access to capital as it's more expensive than borrowing under public ownership."

Shadow business secretary Chuka Umunna said: "Ministers are pushing ahead with this politically-motivated fire sale of Royal Mail to fill the hole left by George Osborne's failed plan."

Business minister Michael Fallon said: "It is the final step to help modernise the business and allow it to invest in the future."

The minister said the Government was aiming for a majority sale so that Royal Mail could access capital markets like any other British business. At least 50% of the business will be sold, although Mr Fallon would not give exact details of the size of the sale and would not estimate how much money will be raised.

Asked about the prospect of a strike by postal workers, he said: "Strike action will not derail the sale. There is no need for strike action - a pay rise of 8.6% over three years has been offered. Teachers and nurses are only getting 1%."